Provided is a system for estimating oxygen saturation by using a photoplethysmography (PPG) signal sensing ring. The system includes a server, wherein the server includes a signal quality classification component configured to classify qualities of a first wavelength PPG signal and a second wavelength PPG signal as good or bad, and an oxygen saturation estimation component configured to estimate oxygen saturation from the first wavelength PPG signal and the second wavelength PPG signal, the PPG signal sensing ring includes a plurality of sensors at different locations, each of the plurality of sensors is configured to measure the first wavelength PPG signal and the second wavelength PPG signal, each of the plurality of sensors includes a first wavelength light source, a second wavelength light source, and a photoelectric conversion device, and the terminal includes a sensor selection component configured to select, from among the plurality of sensors, as a sensor for measuring the first wavelength PPG signal and the second wavelength PPG signal, a sensor that measured a combination of a first wavelength test PPG signal and a second wavelength test PPG signal, which has a highest signal quality from among a plurality of first wavelength test PPG signals and a plurality of second wavelength test PPG signals.